Forced to leave his position at the Sorbonne by the Nazis, Brunschvicg fled to the south of France, where he died at the age of 74. While in hiding, he wrote studies of Montaigne, Descartes, and Pascal that were printed in Switzerland. He composed a manual of philosophy dedicated to his teenage granddaughter entitled Héritage de Mots, Héritage d'Idées (Legacy of Words, Legacy of Ideas) which was published posthumously after the liberation of France. His reinterpretation of Descartes has become the foundation for a new idealism.

Brunschvicg defined philosophy as "the mind's methodical self-reflection" and gave a central role to judgement.

The publication of Brunschvicg's oeuvre has been recently completed after unpublished materials held in Russia were returned to his family in 2001.